1. Field of the Invention
This invention relates to a method of purifying the exhaust gases of an Otto-cycle engine.
2. Description of the Prior Art
A ternary catalyst is employed for purifying the exhaust gases of a known Otto-cycle engine by dissociating their harmful constituents, such as NO.sub.x, CO and HC, and converting them to N, O.sub.2, CO.sub.2 and H.sub.2 O. When the engine has a low cooling water temperature and a low output, however, the loss of heat disables the exhaust gases to reach a temperature of 450.degree. C., and as a consequence, the catalyst loses its activity and allows the harmful substances to be discharged to the atmosphere.
As the low temperature of the exhaust gases is due to the low temperature of the exhaust manifold and pipes of the engine, it has been proposed that the exhaust manifold and pipes be insulated against any substantial loss of heat, or that the exhaust manifold be formed from steel pipes having a smaller wall thickness and therefore a smaller heat capacity, so that it may obtain an elevated temperature more quickly. Neither of these approaches can, however, be expected to produce any satisfactory result, since they are not positive means for raising the temperature of the exhaust gases.
It has also been proposed that the catalyst be positioned immediately after the exhaust manifold, so that the loss of heat occurring to the exhaust gases may be reduced. This approach cannot be expected to produce any satisfactory result, either, but is rather likely to have an adverse effect on the output and thermal efficiency of the engine, since the catalyst adds to the resistance to the flow of the exhaust gases with a resultant increase in the fuel consumption of the engine.
It has further been proposed that a metallic support be used for a metallic catalyst, and supplied with electricity to heat and activate the catalyst. This approach is, however, not practical, since it calls for the use of a large amount of electricity.
As a matter of fact, there is actually no catalyst that can be activated at a temperature which is as low as about 300.degree. C., but it is actually the case that the exhaust gases which are produced by any such engine when it has a low cooling water temperature and a low output, are discharged without being satisfactorily purified.